Fiber devices using grin fiber lenses

ABSTRACT

A mode converter includes first and second optical waveguides and a GRIN fiber lens. The GRIN fiber lens is attached to both the first and the second waveguides.

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/269,586, filed Feb. 17, 2001, and of U.S. ProvisionalApplication No. 60/292,013, filed May 19, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to optical devices and graded refractiveindex lenses.

[0004] 2. Discussion of the Related Art

[0005] A graded refractive index (GRIN) lens has a refractive indexwhose value varies with radial distance from the axis of the lens. Thenon-trivial variation in refractive index causes light refraction andgives the GRIN lens focussing capabilities that are similar to those ofan ordinary lens. Therefore, many optical devices employ GRIN orordinary lenses interchangeably.

[0006] Optical devices use lenses to focus, collimate, or expand lightbeams. FIG. 1 shows a fiber device 10 in which a GRIN fiber lens 11 isfused to a terminal end 12 of an optical fiber 13. The GRIN fiber lens11 expands the light beam emitted by the optical fiber 13. The GRINfiber lens 11 improves the optical coupling between optical fiber 13 andfiber device 15 as compared to the coupling that would otherwise existbetween the fiber 13 and device 15 due to diffraction. The GRIN fiberlens 11 reduces diffraction losses when the optical fiber 13 isoptically coupled to another optical fiber.

[0007] Since the diameter of a light beam varies along the axis of aGRIN lens, the diameter variations provide a measure of the lens'length. The length over which the variations in the beam diameter maketwo complete cycles is known as the pitch of the lens. Typically,lengths of GRIN lens are referred to in multiples of the pitch length,e.g., ½ pitch or ¼ pitch.

BRIEF SUMMARY OF THE INVENTION

[0008] One apparatus embodying principles of the inventions is a modeconverter that reduces losses when waveguides with different propagationmodes are end-coupled. In optical fibers, the forms of the propagationmodes depend on the radial dependence of the refractive index.

[0009] The mode converter includes first and second optical waveguidesand a GRIN fiber lens. The GRIN fiber lens is attached to both the firstand the second waveguides.

[0010] Another apparatus embodying principles of the inventionsend-couples to at least three optical fibers through an optical element.Rather than lenses with curved refractive surfaces, the apparatus usesfiber GRIN lenses attached to the fiber ends for collimation/focusing oflight.

[0011] Another device embodying principles of the inventions includes afiber array an optical element, and a waveguide. In this device, GRINfiber lenses are attached to the fibers of the array. The opticalelement directs light between ones of the fibers and the waveguide

BRIEF DESCRIPTION OF THE FIGURES

[0012]FIG. 1 is a cross-sectional view of a fiber device that uses aconventional GRIN fiber lens to end-couple two optical fibers;

[0013]FIG. 2 is a cross-sectional view of a fiber device in which anoptical fiber is fused to an embodiment of a GRIN fiber lens;

[0014]FIG. 3A shows radial profiles of germanium dopant densities in aconventional GRIN fiber lens and a new GRIN fiber lens;

[0015]FIG. 3B shows radial profiles of refractive indexes for the GRINfiber lenses of FIG. 3A;

[0016]FIGS. 4A and 4B illustrate beam collimation in fiber devices withnew and conventional GRIN fiber lenses, respectively;

[0017]FIG. 5 is a flow chart for a method of fabricating the fiberdevice of FIG. 2.

[0018]FIG. 6A is a cross-sectional view of a mode converter;

[0019]FIG. 6B is a cross-sectional view of a mode converter that uses acompound GRIN fiber lens;

[0020]FIG. 7A is a top view of a 1×2 micro-optical router;

[0021]FIG. 7B is a top view of another topology for a 1×2 micro-opticalrouter;

[0022]FIG. 7C is a top view of a device that optically couples threeoptical fibers;

[0023]FIG. 8 is a cross-sectional view of a 1×N micro-optical router;

[0024]FIG. 9 is a top view of an N×M micro-optical router; and

[0025]FIG. 10 is a cross-sectional view of an optical fiber with anin-line optical device.

[0026] In the Figures, like reference numbers refer to functionallysimilar features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] 1. Grin Fiber Lenses

[0028]FIG. 2 shows an optical fiber device 16 in which an optical fiber17 is end-coupled to a GRIN fiber lens 18, e.g., fused or glued to thefiber 17. The GRIN fiber lens 18 and optical fiber 17 are co-axial andhave similar or equal outer diameters whose values are in the range ofabout 100 microns (μm) to about 135 μm, e.g., 125 μm. The GRIN fiberlens 18 collimates a light beam 19 emitted from the end of the opticalfiber 17 thereby decreasing the numerical aperture below that of a bareoptical fiber. The GRIN fiber lens 18 is also able to focus an incidentlight beam into the end 20 of the optical fiber 17.

[0029] Exemplary optical fibers 17 include single-mode and multi-modefibers.

[0030] Exemplary GRIN fiber lenses 18 have refractive indexes whoseradial profiles differ significantly from those of conventional GRINfiber lenses. The new radial profiles enable decreased numericalapertures and increased Rayleigh ranges for fiber device 16 as comparedto values of the same quantities in conventional fiber device 10 ofFIG. 1. The decreased numerical aperture implies that an appropriatelength GRIN fiber lens 18 would cause less diffraction and a lower powerdensity in emitted light beam 19 than in the light beam 14 emitted byconventional fiber device 10. The increased Rayleigh range implies thatemitted beam 19 is better collimated than the beam 14. The improvedproperties of the emitted beam 19 facilitate transverse alignmentsrequired to end-couple the fiber device 16 to another fiber device (notshown).

[0031] In some embodiments of fiber device 16, GRIN fiber lens 18 has anend face 21 that is angle cleaved to reduce back reflections of lightinto optical fiber 17. In particular, a normal vector to the end-face 21is preferably cleaved at an angle 1°-2° or less with respect to the axisof the GRIN fiber lens 18. This cleave angle is smaller than a typicalcleave angle of about 8° used to lower reflections from its end faceback into the optical fiber (not shown). The beam expansion provided bythe GRIN fiber lens 18 lowers the amount of angle cleave needed toproduce an equivalent reduction in back reflections into the fiber 17.

[0032] The new GRIN fiber lens 18 has a circular core 22 and an annularcladding 24 that surrounds the core 22. In the core 22, the refractiveindex varies with the radial distance from the axis of the GRIN fiberlens 18. In the cladding 24, the refractive index is constant and has alower value than in the core 22. The GRIN fiber lens has an outerdiameter of about 125 μm. The outer diameter is the same as that ofconventional GRIN fiber lens 11 shown in FIG. 1. But, the new andconventional GRIN fiber lenses 11, 18 have different radial refractiveindex profiles due to differences in density distributions of dopantatoms in their cores. Exemplary dopants include germanium (Ge), aluminum(Al), phosphorus (P), and fluorine (F).

[0033]FIG. 3A shows radial profiles 26 and 27 of Ge-dopant densities inconventional GRIN fiber lens 11 and new GRIN fiber lens 18,respectively. In the core 22 of the new GRIN fiber lens 18, theGe-dopant density has a radial profile that is largest on the centralaxis and curved concave downwards. The profile does not have an axialdensity dip, i.e., unlike some conventional GRIN fiber lenses (notshown). The curvature of the radial profile of the Ge-dopant has asmaller average magnitude in the core 22 of the new GRIN fiber lens 18than in the core of conventional GRIN fiber lens 1. In the claddings ofboth the new and conventional GRIN fiber lenses 18, 11, the Ge-dopantdensities are lower than in the fiber cores and are constant withrespect to radial distance from the fiber axes.

[0034] The boundaries between core and cladding, i.e., at radialdistances of R_(c) and R_(c)′, are characterized by abrupt changes inthe Ge-dopant densities and/or radial gradients of the densities. Thecore diameter is larger in the new GRIN fiber lens 18 than inconventional GRIN fiber lens 11, i.e., R_(c)′>R_(c). Increasing the corediameter increases the Rayleigh range of fiber device 16 when a GRINfiber lens 18 of appropriate length is used therein. Exemplaryembodiments of the GRIN fiber lens 18 have an outer diameter of about125 μm. and a core 22 with a diameter of about 85 μm, preferably 100 μmor more, and more preferably 105 μm or more. In some GRIN fiber lenses18, cladding is absent so that the core has a diameter of about 125 μm.

[0035]FIG. 3B shows refractive index profiles 28 and 29 that correspondto the Ge-dopant density profiles 26 and 27 of GRIN fiber lenses 11 and18, respectively. The radial profiles 28, 29 are concave down in thecore 22.

[0036] The radial profiles 28, 29 also show that the new GRIN fiber lens18 has a refractive index whose radial profile has a significantly moregentle variation than in the conventional GRIN fiber lens 11. Aparameter “g” measures the radial curvature of the refractive indexprofile in the core of a GRIN fiber lens. In particular, the parameter gis defined as:${{g = {{- \frac{1}{n_{0}}}\frac{^{2}{P(r)}}{r^{2}}}}}_{r = 0}$

[0037] Here, “r” is radial distance for the axis of the GRIN fiber lens,no is the value of the refractive index on the axis of the GRIN fiberlens, and P(r) is the value of the refractive index at the distance “r”from the axis of the fiber lens.

[0038] The GRIN fiber lens 18 has a refractive index profile that has agentler radial variation over the lens' core. Refractive index profilesof the GRIN fiber lens 18 typically, have radial curvatures that aresmaller in magnitude than those disclosed in Table 1 of “Analysis andEvaluation of Graded-Index Fiber-Lenses”, Journal of LightwaveTechnology, Vol. LT-5, No. 9 (September 1987), pages 1156-1164, by W. L.Emkey et al (EMKEY), which is incorporated by reference herein in itsentirety. Typically, magnitudes of the radial curvature of refractiveindex profile for embodiments of the GRIN fiber lens 18 are, at least,twice as small as values for the same quantity that are disclosed inEMKEY. Exemplary GRIN fiber lens 18 have a “g” that is less than 1.7×10⁻⁶ μm⁻², preferable less than about 0.9×10⁻⁶ μm⁻² and more preferablyless than about 5.0×10⁻⁷ μm⁻². For 125 μm—diameter GRIN fiber lenses 18,values of “g” are selected from the range 1.7×10⁻⁶ μm⁻² to 5.0×10⁻⁷ μm⁻²and preferably in the range 0.9 ×10⁻⁶ μm⁻² to 6.0×10⁻⁷ μm⁻² to providegood beam collimation.

[0039] Exemplary GRIN fiber lens 18 have core index profiles that varyapproximately quadratically in the distance from the lens axis. But,other embodiments of the GRIN fiber lens 18 have non-quadratic indexprofiles.

[0040] Referring again to FIG. 2, the new GRIN fiber lens 18 has a widercore 22 than the conventional GRIN fiber lens 11. The wider core 22 andthe smaller value of the parameter “g” enable the new GRIN fiber lens 18of appropriate length to produce a beam with a wider cross section and alower energy density when used as a beam collimator.

[0041]FIGS. 4A and 4B show light beams 31, 32 emitted by new andconventional fiber devices 16′, 10′ of the types shown in FIGS. 1 and 2.The fiber devices 16′, 10′ have GRIN fiber lenses 18′, 11′ with equalpitches, e.g., {fraction (5/16)} pitch, but different refractive indexprofiles. The new profile in the lens 18′ significantly increases theRayleigh range, RR, of the fiber device 16′ above the Rayleigh range,RR′, of the conventional device 10′. The increased Rayleigh rangeresults from a more gradual beam expansion in the GRIN fiber lens 18′ ascompared to the beam expansion in the conventional GRIN fiber lens 11.In particular, FIGS. 4A and 4B show that making the radial curvature inrefractive index of a GRIN fiber lens smaller than in conventional GRINfiber lenses significantly reduces the divergence of the emitted beamfor a given pitch.

[0042] The Rayleigh range determines the distance range over which anoptical device can couple to a fiber device without substantial losses.The larger Rayleigh range in the new fiber device 16′ makes a larger setof distances available for end-coupling to such a device than areavailable for the conventional fiber device 10′.

[0043] GRIN lenses of equal pitch ordinarily have equal products ofg^(½) times the lens-length. Since the new GRIN fiber lenses 18 havesmaller g-values, the new GRIN fiber lenses 18 are ordinarily longerthan conventional GRIN fiber lenses 11 of equal pitch. The longerlengths make the new GRIN fiber lenses 18 easier to handle, align, andfuse to optical fibers than the conventional GRIN fiber lenses 11. Theincreased lengths also reduce collimation errors associated withcleaving errors that occur during production of the new GRIN fiberlenses 18.

[0044]FIG. 5 is a flow chart for a method 100 of fabricating a GRINfiber lens of doped silica-glass through modified chemical vapordeposition (MCVD). MCVD construction of optical fibers is described inU.S. Pat. Nos. 4,909,816 and 4,217,027, which are incorporated herein byreference in their entirety. The fabrication method 100 includes formingan improved GRIN preform and then, using the improved GRIN preform tomake the GRIN fiber lenses, e.g., GRIN fiber lenses 18 of FIG. 2.

[0045] To form the GRIN preform, layers of silica-glass are depositedinside a silica-glass cladding tube by MCVD (step 102). During the MCVD,a time-varying partial pressure of dopant gases is bled into the gasmixture used to deposit silica-glass on the inside of the cladding tube.Exemplary dopants include Ge, Al, P, and F. Introduction of one or moreof these dopants into the silica-glass changes the refractive index ofthe glass. The partial pressure of dopant gas is varied during the MCVDprocess to produce a non-trivial radial profile of dopant atoms in thefinal silica-glass preform.

[0046] The radial profile in dopant atoms produces a selected radiallygraded refractive index in the final preform. Exemplary profiles for thedopant density and the refractive index have profiles with concavedownward or negative radial curvature. Often, the index profile variesas the square of the distance from the preform's axis in the core of thepreform, e.g., profiles 27, 29 of FIGS. 3A and 3B. Other radial profilesmay be obtained by suitably altering the time-variation of the partialpressure of dopant atoms during the MCVD. Non-quadratic profiles in GRINfibers are capable of reshaping of light beams therein as is known tothose of skill in the art.

[0047] The method 100 includes using the tube produced by the internaldeposition to form the rod-like preform. To form the rod-like preform,heat is applied to partially collapse the tube of doped silica-glass(step 104). In one embodiment, the heating includes making repeatedpasses of the tube through a hot zone of a furnace. The heating isstopped prior to totally blocking the axial channel in the tube withglass.

[0048] After partially collapsing the tube, a silica-glass etchantmixture is passed through the axial channel to remove several layers ofglass from the axis of the tube (step 106). An exemplary gaseous etchantmixture includes C₂F₇, O₂, and Cl₂. Other gaseous etchant mixturesinclude HF. The removed layers have lower dopant concentrations thanadjacent outer layers of silica-glass, because dopants vaporize and arelost through the tube's axial canal during the heating used to collapsethe tube. If these layers with lower dopant densities were not removed,the final preform would have an axial dip in dopant density and acorresponding axial dip in refractive index. The axial dip in refractiveindex interfered the operation of some conventional GRIN fiber lenses.

[0049] After the etching removal of several central layers of glass, thetube is externally heated to finish its collapse to a rod-like preformof doped silica glass (step 108).

[0050] After cooling the preform, etchants are applied to the outersurface to remove a selected thickness of cladding tube from the outsideof the preform (step 110). Removing a portion of the cladding tubeenables subsequent drawing of glass fibers with less or no cladding,e.g., see profiles 27 and 29 in FIGS. 3A and 3B. These thin-clad ornon-clad fibers are advantageous for GRIN fiber lenses, because suchfibers enable an optical beam to expand over a larger portion of thecross section of the final GRIN fiber. Spreading the beam over a largercross section decreases the associated numerical aperture and decreasespower densities so that defects on the end surface of the lens or on thetarget of the emitted beam are less likely to cause component damage.

[0051] Fabrication of GRIN fiber lenses also includes using a standardfiber drawing furnace to draw GRIN fiber from the graded-index preform(step 112). After cooling, one end of the drawn GRIN fiber is fused toone end of a standard fiber, i.e., a fiber with a non-graded index core(step 114). To fuse the GRIN and standard fibers, the ends of the twofibers are heated with an electrical arc or a tungsten filament in anargon environment while the ends are appropriately aligned andpositioned adjacent each other.

[0052] Finally, the GRIN fiber is cleaved to produce an optical lenswith a desired length (step 116). The final attached GRIN fiber lenseshas a pitch of ¼, ½, or any other desired length and is fused to thefiber on which it functions as a beam collimator and expander.

[0053] To reduce reflections from the face of the final fiber deviceback into the fiber, the cleaving is often performed along a directionthat is not perpendicular to the axis of the GRIN fiber. In a non-GRINoptical fiber, cleaving the fiber's end face at an 8 degree angle withrespect to a direction perpendicular to the fiber's axis significantlyreduces back reflections. For a GRIN fiber lens, this cleaving angle canbe reduced to less than 8 degrees from a direction perpendicular to thelens axis to achieve the same reduction in back reflections into anattached optical fiber, e.g., a preferred cleave angle is about 0.5-2degrees.

[0054] The method 100 produces GRIN fiber lenses, e.g. lens 18 of FIG.2, that have lower refractive powers per unit length than conventionalGRIN fiber lenses, e.g., lens 11 of FIG. 1. Thus, the new GRIN fiberlenses are significantly longer than conventional GRIN fiber lenseshaving the same optical power. The longer lenses collimate light betterand are easier to manipulate during device construction. Exemplary GRINfiber lenses with low radial dopant gradients have full pitch lengths ofabout 2, 3, or 4-20 mm.

[0055] The GRIN fiber lens 18 of FIG. 2 can also be made by vapor axialdeposition (VAD), outer vapor deposition (OVD), and sol-gel processesthat are known to those of skill in the art. Such processes are alsoable to avoid creating an axial dip in refractive index in the finalGRIN fiber lens.

[0056] 2. Fiber Devices That Use Grin Fiber Lenses

[0057] Various embodiments provide optical fiber devices that aredescribed below. The various devices described can use eitherconventional GRIN fiber lenses, e.g., lens 11 of FIG. 1, or new GRINfiber lenses, e.g., lens 18 of FIG. 2.

[0058]FIG. 6A shows a mode converter 40 that couples a pair of opticalfibers 36, 38 having different fundamental or higher propagating modes.In some embodiments, the optical fibers 36, 38 have cores of differentdiameters or have refractive index jumps of different sizes acrosscore-cladding boundaries. In the mode converter 40, GRIN fiber lens 43is attached to the ends of the optical fibers 36, 38. In exemplary modeconverters 40, the GRIN fiber lens 43 is either fused directly to theoptical fibers 36, 38 or joined to the fibers 36, 38 by a glue layer(not shown) whose thickness is not greater than the width of the coresof fibers 36, 38.

[0059] Since optical fibers 36, 38 have different core diameters and/orrefractive index jumps, the fibers 36, 38 have propagating modes, e.g.,fundamental modes, with different sizes. Herein, the size of apropagating mode is defined as the mode's full-diameter betweenhalf-maximum amplitude values. Due to the different sizes of thepropagating modes, coupling the optical fibers 36, 38 directly wouldproduce a significant coupling loss of optical energy, i.e., a spliceloss.

[0060] To reduce splice losses, GRIN fiber lens 43 is positioned betweenoptical fibers 36, 38 and is selected to expand the narrower propagatingmode of optical fiber 36 to have a larger diameter that equals that ofthe propagating mode of the optical fiber 38. Designing the GRIN fiberlens 43 to produce the appropriate size conversion entails selecting anappropriate lens length. One of skill in the art would know how toselect the length of GRIN fiber lens 43 based the amount ofmagnification needed to convert the size of the propagating mode of onefiber 36 into that of the propagating mode of the other fiber 38.

[0061] In other embodiments, the mode converter 34 couples a waveguideother than an optical fiber to optical fiber 38.

[0062]FIG. 6B shows a specific embodiment 34′ of the mode converter 34of FIG. 6A. In the mode converter 34′, GRIN fiber lens 43′ is a compoundlens made of a sequence of GRIN fiber lens elements 43A, 43B. The firstelement 43A is fused directly to the end of optical fiber 36, and thelast element 43B is fused directly to the end of optical fiber 38.Exemplary GRIN elements 43A and 43B are fused together and havedifferent refractive index profiles and lengths. The lengths and indexprofiles of the two lens elements 43A, 43B are selected to betteroptically couple the fibers 36, 38. In some embodiments, the first GRINelement 43A expands the light beam emitted by fiber 36, and the secondelement 43B focuses the beam waist to the size of the propagating modein the fiber 38.

[0063]FIG. 7A shows a 1×2 micro-optical router 46. The router 46includes an input optical fiber 48, output optical fibers 50, 52, and amovable reflector 54 for directing light from the input fiber 48 to aselected one of the output fibers 50, 52. The terminal ends of theoptical fibers 48, 50, 52 are fused to GRIN fiber lenses 49, 49′, 49″,e.g., identical GRIN fiber lenses. The GRIN fiber lens 49 functions tocollimate or focus the emitted light beam from fiber 48. The GRIN fiberlenses 49′, 49″ function to collect light and couple the collected lightinto the associated optical fibers 50, 52. The output optical fibers 50,52 are located so that the waist of the beam emitted by the inputoptical fiber 48 is at the midpoint of the optical path between theinput and output optical fibers 48, 50, 52. The reflecting surface ofreflector 54 is located at the beam waist to within about a Rayleighrange when positioned to reflect light to the output optical fiber 50.

[0064] To select a routing, reflector 54 is moved in or out of the pathof the light beam emitted by optical fiber 48. The reflector 54 is fixedto a micro-electro-mechanical (MEM) device 56 that moves the reflector54 in and out of the beam's optical path in response to electricalsignals applied to the MEM device 56.

[0065] The GRIN fiber lenses 49, 49′, 49″ improve beam collimation andcollection so that terminal ends 58, 60, 62 can be separated bydistances that are large enough to enable insertion and removal ofreflector 54 in routing region 64. In embodiments of router 46 based onthe new GRIN fiber lenses 18 of FIGS. 2, 3A-3B, and 4A, better beamcollimation enables distances between terminal ends 58, 60, 62 to be aslarge as about 9 mm. For these large inter-fiber distances, the GRINfiber lenses 49, 49′, 49″ reduce optical coupling losses to less thanabout 0.5 decibels (dB) and preferably to less than about 0.2 dB-0.05dB. However, larger inter-fiber spaces involve more serious fiber devicealignment issues.

[0066] In some embodiments, the micro-router 46 has an overall size, S,that is much smaller than the overall size of an analogous router inwhich the GRIN fiber lenses 49, 49′, 49″ are replaced by conventionallenses with curved refractive surfaces. The lenses with curvedrefractive surfaces have larger diameters than the GRIN fiber lenses 49,49′, 49″. The larger lens diameters require positioning the ends of theinput and output fibers at larger separations in such a router than inthe micro-router 46. The lenses with curved refractive surfaces alsotypically produce larger diameter collimated beams in the routing regionthan the fused GRIN fiber lenses 49 of micro-router 46. The larger beamdiameters necessitate a larger reflective surface on the routeringreflector of the router whose lenses have curved refractive surfacesthan would be needed on the reflector 56 of the micro-router 46.

[0067] In some embodiments of micro-router 46, the distance, S,characteristic of separations between GRIN lenses 49, 49′, 49′ has avalue in the range of about 1-3 times the fiber diameter to about 1-3times the Rayleigh range, e.g., less than about 1 mm. In theseembodiments, the small size of the region 64 between the lenses 49, 49′,49″ is achieved in part, because diameters of the attached GRIN fiberlenses 49, 49′, 49″ are small and in part, because the reflectivesurface on reflector 54 has a small beam acceptance window. Theacceptance window for reflecting the input beam can be less than thefiber diameter, because the GRIN fiber lens 49 produces a beam waistthat is smaller than the diameter of fiber 48. Both the small diameterGRIN fiber lenses 49, 49′, 49″ and the smallness of reflector 54 enablethe router 46 to be much smaller than routers that use lenses withcurved refractive surfaces.

[0068]FIG. 7B shows an alternate embodiment 46′ of the router 46 shownin FIG. 7A. In router 46′, the fibers 48, 50, 52 are adjacent andlocated in a linear array 68. A single rotatable reflector 56′, e.g., aMEMS controlled reflector, selectively routes light from the fiber 48 toeither the fiber 50 or the fiber 52. In some embodiments, the axes thefibers 50 and 52 are slightly tilted with respect to the axis of thefiber 48 to insure that light from the reflector 56′ parallel to theaxis of the fibers 50, 52.

[0069] Arranging the fibers 48, 50, 52 in array 68 makes the width ofthe router 46′ roughly equal to the width, W, of the array 68. The smalldiameters and fine collimation of GRIN fiber lenses 49, 49′, 49″ enablepacking the fibers 48, 50, 52 closely in the array 68. Thus, embodimentsof the router 46 can have a width, W, that is much smaller than thewidth of a similar-form router in which lenses with curved refractivesurfaces replace the GRIN fiber lenses 49, 49′, 49″.

[0070]FIG. 7C shows an embodiment of an optical device 46″ that couplesthree optical fibers 48, 50, 52 based on light polarization, lightwavelength, or relative fiber position. The optical fibers 48, 50, 52have attached GRIN fiber lenses 49, 49′, 49″ that collimate and collectlight. The device 46″ includes an optical element 54′ that transmitslight between the optical fibers 48, 50, 52, e.g., in a manner thatdepends on polarization or wavelength. In various embodiments, opticaldevice 54′ includes a polarizing beamsplitter, a grating, an opticalcirculator, or a wavelength selective reflector such as a Bragg grating.

[0071]FIG. 8 shows a 1×N micro-optical router 70 that includes an inputoptical fiber 72, an output array 73 of N output optical fibers 74 ₁-74_(N), and a reflector 76. The optical fibers 72, 74 ₁-74 _(N) aresingle-mode fibers to which terminal GRIN fiber lenses 77 ₀-77 _(N) havebeen fused. The light beam 78 from the input optical fiber 72 intersectsthe reflector 76 near the waist of the beam 78, i.e., within ½ aRayleigh range.

[0072] Exemplary reflectors 76 include mirrors that move or rotate anddiffraction gratings that reflect light in a wavelength dependentmanner. For example, the router may be a spectrally sensitivedemultiplexer for a wavelength division multiplexed network.

[0073] The GRIN fiber lenses 77 ₀-77 _(N) expand and collimate the lightbeam 78 of the input optical fiber 72 and focus the light beam 78 intothe output optical fibers 74 ₁-74 _(N). Due to the GRIN fiber lenses 77₀-77 _(N), the output array 73 of optical fibers 74 ₁-74 _(N) and inputoptical fiber 72 can be separated by an optical path that is long enoughto enable insertion of bulk reflector 76 into the path withoutsignificant coupling losses. For the router 70 coupling losses aretypically less than about 0.5 dB-0.2 dB and preferably less than about0.1 dB.

[0074] In micro-optical router 70, GRIN fiber lens 77 ₀ focuses the beamfrom fiber 72 onto a reflective acceptance window on the reflector 76.Perpendicular to direction D, the diameter of the acceptance window isless than the fiber diameter. Also, the use of the GRIN fiber lenses 77₀-77 _(N) enables an increased fiber packing density in the array 73without interference between light beams reflected towards differentones of the fibers 74 ₁-74 _(N). Finally, the use of GRIN fiber lens 77₀ enables the acceptance window and overall size of reflector 76 to besmaller than that of the reflector that would otherwise be needed in arouter using lenses curved refractive surfaces (not shown). Thus, usingthe GRIN fiber lenses 77 ₀-77 _(N) enables greater miniaturization inmicro-router 70 than in a fiber router based on lenses with curvedrefractive surfaces.

[0075] Other embodiments use the GRIN fiber lens 18 of FIG. 2 toconstruct Nxl routers (not shown) by methods that would be obvious toone of skill in the art in light of the above-disclosure. For example, a2×1 router can be constructed by exchanging designations of input andoutput for fibers 48, 50, 52 in 1×2 micro-router 46 of FIG. 7A.

[0076]FIG. 9 is a top view of an N×M optical router 80. The router 90includes an array 81 of N input optical fibers, 82 ₁-82 _(N), and anarray 83 of M output optical fibers, 84 ₁-84 _(M). The fibers 82 ₁-82_(N), 84 ₁-84 _(M) have GRIN fiber lenses 85 ₁-85 _(N), 86 ₁-86 _(M)fused to terminal ends thereof. The GRIN fiber lenses 85 ₁-85 _(N), 86₁-86 _(M) provide beam collimation and collection functions analogousthose previously described in relation to GRIN fiber lenses 49, 49′, 49″of FIG. 7A. Between the input and output fibers 82 ₁-82 _(N), 84 ₁-84_(M) are banks 87 _(F), 87 _(R) of fixed and routing reflectors, 88_(F1)-88 _(FN), 89 _(R1)-89 _(RN). Exemplary reflectors 89 _(R1) 89_(RN) include wavelength-selective reflectors, e.g., gratings, andwavelength insensitive reflectors. Properly aligning the reflectors 88_(R1)-88 _(RN) routes light from individual ones of the input fibers 82₁-82 _(N) to selected ones of the output fibers, 84 ₁-84 _(M). Thereflectors 88 _(R1)-88 _(RN) are operated by MEMs devices 89 ₁-89 _(N)and have acceptance windows for input beams whose diameters are smallerthan the inter-fiber spacing, IFS, of array 81.

[0077] By using attached GRIN fiber lenses 85 ₁-85 _(N), 86 ₁-86 _(M)the fiber packing densities in the arrays 81, 83 can be increased abovefiber packing densities of an N×M fiber router in which lenses withcurved refractive surfaces (not shown) replace the GRIN fiber lenses 85₁-85 _(N), 86 ₁-86 _(M) of FIG. 9. Similarly, sizes of reflectivesurfaces of reflectors 88 _(F1)-88 _(FN), 89 _(R1)-89 _(RN) in therouter 80 are smaller than sizes of reflective surfaces of reflectors inrouters based on lenses with curved refractive surfaces, because thebeam diameters produced by the GRIN fiber lenses 85 ₁-85 _(N) are small.Both effects enable the new N×M to be smaller than an N×M router basedon lenses with curved refractive surfaces.

[0078]FIG. 10 shows a micro-optical device 90 that is located in-linebetween ends 91, 93 of optical fibers 92, 94. Exemplary micro-opticaldevices 90 include wavelength-sensitive add/drop modules, polarizers,polarization rotators, one-way optical isolators, and controllableoptical attenuators. The ends 91, 93 of the optical fibers 92, 94 arefused to GRIN fiber lenses 96, 98. The GRIN fiber lens 96 collimateslight emitted by the optical fiber 92. The GRIN fiber lens 98 focusesreceived light into the optical fiber 94. The micro-optical device 90has an approximate thickness, d, that is not greater than the Rayleighrange associated with the GRIN fiber lenses 96, 98. For such athickness, the GRIN fiber lenses 96, 98 reduce diffraction-relatedcoupling losses.

[0079] Other embodiments of the invention will be apparent to thoseskilled in the art in light of the specification, drawings, and claimsof this application.

What is claimed is:
 1. An apparatus for mode converting, comprising:first and second optical waveguides; and a GRIN fiber lens attached toboth the first and the second waveguides.
 2. The apparatus of claim 1,wherein the waveguides are fused or glued to the GRIN fiber lens.
 3. Theapparatus of claim 2, wherein the first and second waveguides are firstand second optical fibers, respectively.
 4. The apparatus of claim 3,wherein the first fiber has propagation modes with different sizes thanthe second fiber.
 5. The apparatus of claim 3, wherein the lens has amagnification, the magnification times the size of a fundamentalpropagation mode of the first fiber being about equal to the size of afundamental propagation mode of the second fiber.
 6. The apparatus ofclaim 3, wherein the first and second fibers have cores with differentdiameters.
 7. The apparatus of claim 3, wherein each fiber has a coreand a cladding; and a discontinuity in refractive index across aninterface between the core and cladding, the discontinuities beingdifferent across the interfaces of the first and second fibers.
 8. Theapparatus of claim 3, wherein the GRIN fiber lens comprises a series ofconnected GRIN fiber lenses; the first GRIN fiber lens of the seriesbeing attached to the first fiber and the last GRIN fiber lens of theseries being attached to the second fiber.
 9. The apparatus of claim 1,wherein the GRIN fiber lens has a core with a graded refractive indexprofile, the profile having a radial second derivative whose averagemagnitude is less than about 2.4×10⁻⁵ microns⁻² in the core.
 10. Anapparatus, comprising: first, second, and third optical fibers; first,second, and third GRIN fiber lenses attached to the first, second, andthird optical fibers, respectively; and an optical element configured tooptically couple the first, second, and third optical fibers.
 11. Theapparatus of claim 10, wherein free ends of the first, second, and thirdGRIN fiber lenses have separations of less than about 1 millimeter. 12.The apparatus of claim 10, further comprising: a MEM device; and whereinthe optical element is a moveable reflector whose position ororientation is controlled by the MEM device.
 13. The apparatus of claim10, wherein the optical element includes one of an optical circulator, apolarization-selective splitter, and a wavelength-selective reflector.14. The apparatus of claim 10, wherein a free surface of one of the GRINfiber lenses is cleaved at an angle of less than 8 degrees from a planewhose normal is the lens' axis.
 15. An optical apparatus, comprising: anarray of at least three optical fibers having attached GRIN fiberlenses; and an optical device configured to direct light betweenselected ones of the GRIN fiber lenses of the array and another opticalwaveguide.
 16. The apparatus of claim 15, wherein the other waveguide isan optical fiber having an attached fiber GRIN lens.
 17. The apparatusof claim 15, wherein a free surface of one of the GRIN fiber lenses iscleaved at an angle of less than 8 degrees from a plane whose normal isthe lens' axis.
 18. The apparatus of claim 16, wherein the opticaldevice is configured to present an acceptance window for light from oneof the fibers, the window having a diameter not greater than thediameter of one of the optical fibers.
 19. The apparatus of claim 15,wherein free ends of the GRIN fiber lenses have separations of less thanabout 1 millimeter.
 20. The apparatus of claim 15, wherein the opticaldevice includes one of a reflector, a polarization-sensitive splitter,and a wavelength-sensitive reflector.
 21. The apparatus of claim 16,further comprising: a second array of optical fibers having attachedGRIN fiber lenses, the waveguide being one of the fibers in the secondarray; and wherein the optical device includes a plurality of elementscapable of routing light from the fibers of the first array to thefibers of the second array.
 22. The apparatus of claim 21, wherein atleast one of the elements is one of a polarization-sensitive splitterand a wavelength-sensitive reflector.
 23. The apparatus of claim 21,wherein free ends of the GRIN fiber lenses of the array have separationsof less than about 1 millimeter.
 24. The apparatus of claim 21, whereina free surface of one of the GRIN fiber lenses of the array is cleavedat an angle of less than 8 degrees from a plane whose normal is thelens' axis.
 25. The apparatus of claim 21, wherein the optical device isconfigured to present an acceptance window for light from one of thefibers, the window having a diameter not greater than the diameter ofone of the optical fibers.